It seems that NASA has accumulated a lot of indirect evidence for life outside Earth. Of course the prime candidate for extraterrestrial life is Mars. That Mars had lots of liquid water and has detectable methane in the atmosphere and these arguesfor the presence of life or past existence of life. The NASA Viking experiments in 1976, were designed to detect microbial life but the results were inconclusive. The robots landed on Mars but because of the strange soil chemistry, all the biochemistry experiments came out inconclusive.
So astro evolutionary biologist Peter Ward argues that we must accept as very probable that life is common at least in the solar system and even more probable in the cosmos. Much of this life is not complex. But if we accept that there is extraterrestrial life then we have to revise our understanding of what life is.
In biology class we are taught that living things have or can 1) DNA and RNA, 2) are made of cells, 3) reproduce, 4) develop and grow, 5) metabolizes, 6) evolves and 7) be autonomous. These are sure signs of something that's alive. But these define life on Earth. Now Peter Ward calls Earth Life as Terroa.
Viruses seem not to fall into any of these 7 criterion except perhaps they can evolve. Now are viruses alive?
Ward thinks they are if we use the Astrobiologist's definition. The astrobiologist says that "life is a chemical system that can evolve in a Darwinian-Wallacean sense." Viruses do have DNA or RNA and can evolve.
Terroans at present are CHON life forms (carbon, hydrogen,oxygen, nitrogen). But Ward shows us the possibility of other life forms that have other solvents as a medium for metabolism. CHON life requires water. But Ward proposes a hypothesis that there could be silicon based life that requires ethane as a solvent (such as what is found on Saturn's big moon, Titan). Ward's hypothesis says that we don't need Earth like environments for life to evolve.
Ward also says that our technology has come to a point that we can be able to create our own tailor made viruses. If we adopt Ward's definition of life then we have become God! But early in Earth's history life probably was dominated by viruses of the RNA kind. If viruses can live in inhospitable environments, then we can expect something similar on many planets.
Ward who begins his Astrobiology book "Life as we not know it" starts with the Cantina scene in "Star Wars". The Star Wars Universe is a prescient work of sci-fi. It shows the cosmos full of biodiversity and not just stereotypical aliens. Even if in the most fanous barroom scene in the galaxy, Han Solo shoots the bounty hunter, the message goes through: The cosmos is biodiverse.
But according to Ward, aliens are under our noses or in our noses and they give us sneezing bouts.